It is well known in the art of using semi-permeable membranes to separate air, that said membranes in general will preferentially pass oxygen, and a detailed account of the use of semi-permeable membranes for the separation of gases is provided in an article by I. Cabasso, in the Encyclopedia of Polymer Science and Engineering, Vol. 9, pg. 564 by John Wiley & Sons, N.Y. 1987.
However, a need exists for semi-permeable membranes which will enable preferential passing of N.sub.2, for purposes where N.sub.2 enrichment is useful and where cryogenic processes for obtaining nitrogen enrichment are sought to be avoided.